In communication channels, data must be transmitted through the channel reliably. Data is represented as a sequence of bits, which each bit taking a value of zero or one. In most communication channels, two major components ensure the reliability of the data: a detection channel (or detector) and an error correction code (ECC). The detector receives an analog waveform from the channel, converts the analog waveform to a digital waveform, and then converts the digital waveform into ones and zeros. The ones and zeros are grouped in contagious subsequence of bits known as symbols. The number of bits in a symbol is determined as a parameter of the ECC and is typically a small number, such as ten. The data symbols are transmitted to an ECC decoder, where erroneous symbols are corrected, assuming that the number of symbols that the ECC has been designed to correct has not been exceeded.
A simple ECC code is based on parity. A parity bit is added to a group of data bits, such as a data word, and has a logic state that is selected to make the total number of ones (or zeros) in the data word either even or odd. The original data word is then transmitted to the channel along with the additional parity bit as a modified data word or “ECC symbol”. The ECC symbol is received from the channel and ECC decoder checks the parity of the ECC symbol against an expected value. If the parity is correct, the ECC detection circuit assumes there are no bit errors. If the parity is incorrect, the ECC detection circuit assumes there is an error in the transmitted data.
More complex ECC codes have also been used for enabling not only detection of additional errors but also correction of some of the detected errors. For example, a single-error correction, double-error detection (SEC-DED) Hamming code adds enough additional parity bits to enable the detection circuit to detect and correct any single-bit error in a data word and detect two-bit errors. Other types of error correction codes include convolution (tree) codes and block codes. In these types of ECC codes, one or more data words are divided into blocks of data, and each block of data is encoded into a longer block of data known as an ECC symbol. With convolution codes, the encoding of one block of data depends on the state of the encoder as well as the data to be encoded. For example, Reed Solomon ECC codes correct symbols (groups of bits), not bits.
A new method of detection is now being considered for use in some communication channels, including hard drive and storage applications. That method is called “iterative” (or “turbo”) decoding, because the data is processed multiple times in the detector. In a typical iterative decoder, special coding (parity and interleaving are two of several options) is introduced before the data is transmitted to the channel. When the data is received from the channel, the data runs through a “soft decoder”, which produces quality “soft” information about each bit decision it makes. The soft decisions are transferred to a block that resolves the parity based on the hard and soft information. This step is often implemented with a technique called “message passing.” Once the message passing is complete, both the soft and hard information have been altered and hopefully improved. This updated information is passed back to the soft decoder where the signal is detected again. Finally, the hard and soft detector output is sent back to the parity resolver, where the hard and soft information is once again improved. This iteration process may continue any number of times. Practically, the number of iterations is limited by the time that system has to deliver the data to the user. The result is an increased confidence or reliability of the detected data.
When ECC is used in combination with an iterative type of encoding/decoding system, the ECC symbols are typically generated before the iterative or turbo code is applied. Although ECC codes enable the detection and correction of some errors that are not detected by the iterative or turbo detector, further coding of the data bits before transmission through the channel can limit the effectiveness of the ECC codes. Improved coding and decoding techniques are therefore are desired.
Embodiments of the present invention provide solutions to these and other problems and offer other advantages over the prior art.